This invention relates to a method and apparatus for the heat treatment of solid and fluid materials in a furnace with, for example, hot air.
Heretofore, an apparatus in which heated fluid is introduced, either from above or below, into a fluid, movable, or stationary bed of material to be treated has been used as a combustion furnace, a cracking furnace, a burning furnace, a carbon activating furnace, a roasting furnace, and a recovery furnace.
A difficulty with such apparatuses is that light particles splash out of the bed. The light particles are those initially contained in the bed material to be treated, and those created by the physical and chemical interaction between the bed material and the heated fluid. It is very difficult to retain these light particles in the bed and to control their interaction in the bed. Accordingly, heat treatment furnaces and devices for separating and recovering solid matter from the gas have been separately provided, which is disadvantageous in terms of installation cost and size.
As an environmental problem, and for minimizing air pollution, there is a strong demand for the heat-treatment of minute solid particles, such as soot and waste materials generated in industrial plants and treatment facilities, and sludge which includes such particles. For example, a great quantity of incomplete combustion particles are entrained in the gas discharged through the funnel of a boiler or an incinerator. This is one of the basic causes of air pollution. Also, large quantities of carbon dust are recovered from the collecting sections of electric generator plant boilers, which include 50 - 98% carbon containing ash such as ammonium sulfate, metal, silica, and alumina. The heat treatment of such carbon dust is necessary. All of the metal machining industries employ some type of metal polishing process, in which an abrasive and metal powders become mixed in an oil and water sludge. It is necessary to heat-treat such sludge to recover the metal and the abrasive powder, in order to prevent air pollution thereby.
There are many kinds of industrial sludge, such as metal sludge, sludge created in the food industry, paper sludge, and sludge obtained by polishing quartz.
Asbestos is employed in a number of industrial fields, such as for gaskets, packings, electrolytic diaphragms, brakes, heat insulators and heat resisting materials. In the processes of preparing the raw material, and in molding, cutting and polishing it, a great quantity of waste is created.
It is also necessary to recycle industrial molding sand, which includes about 1% by weight of phenol resin as an adhesive. In addition, chemicals such as ammonium sulfate, hydrogensulfate alkali metal salt, dithionic acid, imidodisulfonate, etc. produced in desulfurization and denitrization processes by the boilers and furnaces must be thermally cracked.
Heretofore, most asbestos containing waste materials have been abandoned. More recently, however, the discarding of these waste materials has been prohibited since it is now known that asbestos is a major cause of lung cancer.
Thus, there is a great demand for a compact and efficient heat treating apparatus suitable for treating minute solid particles.
Several methods for separating and collecting soot from gas and burning it again have been proposed in the art. Since, soot has a low specific gravity, however, it is difficult to centrifugally separate and collect it. It is also difficult to collect soot with an electrical precipitator, because its electrical resistance is very low. Thus, while soot is readily charged, upon collection its polarity becomes the same as that of the electrode, as a result of which the soot is released and returned into the atmosphere.
In addition, the prior art methods of burning soot suffer from the disadvantage that the apparatuses required are unduly large, and it is difficult to achieve complete combustion in them.
As a result, soots and sludges such as metallic sludge, food sludge, paper sludge, and quartz polishing sludge, and minute solid particles such as molding sand and asbestos, have not been recycled or utilized again in the past, but have merely been discarded. A method of utilizing such waste materials has not yet been proposed.